High performance aluminum nanocomposites

ABSTRACT

The high performance aluminum nanocomposites are formed by a combination of mechanical alloying and Spark Plasma Sintering (SPS) in order to obtain reinforced nanostrutured aluminum alloys, The nanocomposites are formed from aluminum metal reinforced with silicon carbide (SiC) particulates, wherein the SiC particulates have a particle diameter between about 20 and 40 nm. The nanocomposites are prepared by mixing aluminum-based metal, e.g., Al-7Si-0.3Mg, (Al=92.7%, Si-7% and Mg=0.3%), with SiC nanoparticles in a conventional mill to form a uniformly distributed powder, which is then sintered at a temperature of about 500° C. for a period up to about 20 hours to consolidate the silicon carbide particulates in order to obtain the reinforced aluminum metal-based silicon carbide nanocomposite.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aluminum alloys, and particularly tohigh performance aluminum nanocomposites that are based on aluminum (Al)reinforced with nano-sized silicon carbide (SiC) particles.

2. Description of the Related Art

The commercially available aluminum-based alloys, in particular Al—Si—Mgalloys, are used extensively in the automotive industry in variouslocations, such as cylinder blocks, cylinder heads, pistons, and valvelifters. Under normal applications, Al—Si—Mg alloys are processed inboth cast and wrought forms. They are age-hardenable and are routinelyheat treated to T6 condition to develop adequate strength. Differentindustrial applications have called for more advanced processing ofaluminum-based alloys, especially Al—Si—Mg alloys, because they arebeing utilized in multiple vital applications. This is primarily due totheir improved corrosion resistance and to their high specific strength.These Al—Si—Mg alloys are aggressively replacing steels in variousindustries in order to have lighter components with improved properties.However, there has been a limit to the possible improvement of theirproperties using conventional processing routes, and research effortshave been directed to boosting their performance by adding a secondphase particle in different compositions and sizes. Also, theconventional Al—Si—Mg alloys used in the automotive industry havelimited capabilities, and their properties can't be stretched beyondcertain limits. Indeed, most conventional processes that utilize normalconsolidation procedures fail to retain the nano-crystalline structuretill the final product is reached.

Thus, high performance aluminum nanocomposites solving theaforementioned problems are desired.

SUMMARY OF THE INVENTION

The high performance aluminum nanocomposites are formed by a combinationof mechanical alloying and Spark Plasma Sintering (SPS) in order toobtain reinforced nanostructured aluminum alloys. The nanocomposites areformed from aluminum metal reinforced with silicon carbide (SiC)particulates, wherein the SiC particulates have a particle diameterbetween about 20 and 40 nm. The nanocomposites are prepared by mixingaluminum-based metal, e.g., Al-7Si-0.3 Mg, (Al=92.7%, Si=7% andMg=0.3%), with SiC nanoparticles in a conventional mill to form auniformly distributed powder, which is then sintered at a temperature ofabout 500° C. for a period up to about 20 hours to consolidate thesilicon carbide particulates in order to obtain the reinforced aluminummetal-based silicon carbide nanocomposite.

These and other features of the present invention will become readilyapparent upon further review of the following specification anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a scanning electron microscope (SEM) micrograph ofAl-7Si-0.3Mg+20 wt. % SiC powder blend milled for 5 hours.

FIG. 1B shows a SEM micrograph of Al-7Si-0.3Mg+20 wt. % SiC powder blendmilled for 12 hours.

FIG. 1C shows a SEM micrograph of Al-7Si-0.3Mg+20 wt. % SiC powder blendmilled for 20 hours.

FIG. 2 is a transmission electron microscope (TEM) bright field image ofa powder sample of an Al—Si—Mg alloy with 5% SiC milled for 20 hours,showing a crystalline shape with a diameter of about 100 nm.

FIG. 3 is a graph showing the reduction of crystallite size of anAl—Si—Mg alloy reinforced with 5% SiC and with 20% SiC as a function ofmilling time.

FIG. 4 is a graph showing shows the per cent densification as a functionof the spark plasma sintering (SPS) temperature for Al—Si—Mg samplesreinforced with 0%, 5%, 12%, and 20% SiC, respectively.

FIG. 5 is a graph showing the hardness as a function of the SPSsintering temperature for Al—Si—Mg samples reinforced with 0%, 5%, 12%,and 20% SiC, respectively.

Similar reference characters denote corresponding features consistentlythroughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The high performance aluminum nanocomposites are formed by a combinationof mechanical alloying and Spark Plasma Sintering (SPS) in order toobtain reinforced nanostructured aluminum alloys. The nanocomposites areformed from aluminum metal reinforced with silicon carbide (SiC)particulates, wherein the SiC particulates have a particle diameterbetween about 20 and 40 nm. The nanocomposites are prepared by mixingaluminum-based metal, e.g., Al-7Si-0.3Mg, (Al=92.7%, Si=7% and Mg=0.3%)or Al-12Si-0.3Mg, with SiC nanoparticles in a conventional mill to forma uniformly distributed powder, which is then sintered at a temperatureof about 500° C. for a period up to about 20 hours to consolidate thesilicon carbide particulates in order to obtain the reinforced aluminummetal-based silicon carbide nanocomposite.

As used herein the term “mechanical alloying” refers to a solid statepowder processing technique involving repeated cold welding, fracturingand re-welding of powder particles in a high energy ball mill, which maybe a planetary ball mill. As used herein, the term “spark plasmasintering (SPS)” refers to a sintering technique in which a pulsed DCcurrent directly passes through the die having the powder so that thepowder is consolidated with the application of high temperature.

Nano-silicon carbide (SiC-β) particles of sizes between 20-40 nm andwith high purity (<99%) are used in order to reinforce two conventionalaluminum-based alloys, namely, Al-7Si-0.3Mg (Al=92.7%, Si=7% andMg=0.3%) and Al-12Si-0.3Mg (Al=87.7%, Si=12%, Mg=0.3%)(all percentagesby weight). The aluminum-based alloys were obtained in powder form andhad particle sizes averaging 40 nanometers.

The nanoeomposites may be mixed together through milling the twoconstituents (SiC and aluminum-based alloys) in a planetary mechanicalmill (Fritsch Pulverisette 5) at different parameters. Typically, thenano-sized silicon carbide (SiC) particles were added to variouscompositions at 5%, 12% and 20% by weight and then milled at 200 rpmwith the aluminum-based alloys for different periods of time up to amaximum of 20 hours. FIGS. 1A-1C display the scanning electronmicroscope (SEM) micrographs of Al-7Si-0.3Mg+20 wt % SiC powderblend-milled for 5 hours, 12 hours, and 20 hours, respectively. Thedistribution of the augmentation was evaluated at different millingtimes in order to ensure optimum distribution that can facilitate themaximum improvement in properties. Energy Dispersive Spectroscopy (EDS)indicated that the reinforcement phase was distributed uniformly usingmechanical milling, especially at prolonged milling time of 20 hours.Direct transmission electron microscopy (TEM) of the samples milled for20 hours shows that the final crystalline size was about 100 nm. FIG. 2shows a TEM bright field image of a powder sample with 5% SiC milled for20 hours, indicating a crystallite with a diameter of about 100 nm.Additionally, FIG. 3 shows the reduction of crystallite size as millingprogresses. It is apparent that the addition of higher percentages ofSiC further refined the structure of the mixture.

After the powders were processed using mechanical alloying, they weresubjected to consolidation via spark plasma sintering (SPS) so that thefinal aluminum alloy product could be obtained. The use of SPS proved tobe an excellent choice as the consolidated materials didn't lose theirdeveloped nanostructure due to the heat associated with consolidation,and this is primarily due to the fact that SPS induces massive heatingin a very short period of time, which doesn't allow for grain growth totake place. Consolidation of the powder was carried out using threedifferent temperatures of 400° C., 450° C., and 500° C. respectively inorder to ascertain the optimum processing conditions. FIG. 4 shows thevariation of densification for SiC/Al-7Si-0.3Mg alloy against the SPSsintering temperature, and FIG. 5 shows the hardness against the SPSsintering temperatures and silicon carbide concentration. TheSiC/Al-7Si-0.3Mg alloy has an average hardness as measured by Vickersindenters of not less than 40. Typically, the hardness is in the rangeof 40 to 70. The densification and the hardness parameters increase witha concomitant increase in the sintering temperature.

The process for making the nanocomposite resulted in the best alloyafter about 20 hours of milling and sintering at about 500° C. Themilling process of the invention allows for attaining a crystallite sizebelow 100 nm. The best performing alloy was found to be the alloycontaining 20% of nano-SiC consolidated at 500° C. The improvement inproperties was due to the retention of the nanostructure, as the use ofSPS proved to be useful in this regard. The above results were obtainedwith the Al-7Si-0.3 Mg aluminum alloy. Results obtained for theAl-12Si-0.3Mg aluminum alloy exhibited a more or less similar trend.

Additionally, the method unexpectedly achieves nano-structures of thereinforced aluminum SiC alloys. Generally, bringing materials in thenano-structure regime introduces several improvements into the alloysproperties such as improved hardness and strength. The nanostructure ofthe alloy is reached during the mechanical alloying stage of the millingprocess, whereas the spark plasma sintering preserves thenano-structures. Most conventional processes that utilize normalconsolidation procedures fail to retain the nano-crystalline structuretill the end when the final product is reached, while in the presentmethod results in a reinforced alloy material retaining thenano-crystalline structure.

The high performance nanocomposite alloys that are based on aluminum andreinforced with nano-SiC particulates can be useful especially inautomotive industry because they provide increased performance inaluminum-based alloys as commonly used in cylinder blocks, cylinderheads and pistons etc. The above method can also be used to scale upproduction with high quality products, if adequate precautions are beingconsidered.

It is to be understood that the present invention is not limited to theembodiments described above, but encompasses any and all embodimentswithin the scope of the following claims.

We claim:
 1. A high performance aluminum nanocomposite, comprising analuminum alloy reinforced with uniformly distributed silicon carbideparticles, wherein the silicon carbide particles have a diameter betweenabout 20 and 40 nm.
 2. The high performance aluminum nanocomposite ofclaim 1, wherein the aluminum alloy comprises Al-7Si-0.3Mg, having acomposition of about 92.7% by weight of Al, 7% by weight of Si and 0.3%by weight of Mg.
 3. The high performance aluminum nanocomposite of claim1, wherein the aluminum alloy comprises Al-12Si-0.3Mg, having acomposition of about 87.7% by weight of Al, 12% by weight of Si and 0.3%by weight of Mg.
 4. The high performance aluminum nanocomposite of claim1, wherein the nanocomposite is nano-crystalline in structure, having acrystalline size of up to 100 nm.
 5. The high performance aluminumnanocomposite of claim 1, wherein the nanocomposite has an averagehardness as measured by Vickers indenters of not less than
 40. 6. Thehigh performance aluminum nanocomposite of claim 1, wherein the siliconcarbide nanoparticles comprise between 5 wt % and 20 wt % of thenanocomposite, the balance being the aluminum alloy.
 7. A method ofmaking a high performance aluminum nanocomposite, comprising the stepsof: (a) mixing an aluminum-based metal alloy with silicon carbidenanoparticles by mechanical alloying in a ball mill to form a uniformlydistributed powder having a nano-crystalline structure; (b) sinteringthe powder to consolidate the powder to obtain a reinforced aluminumalloy nanocomposite, whereby the mechanically alloyed powder retains thenano-crystalline structure after sintering.
 8. The method of making ahigh performance aluminum nanocomposite according to claim 7, whereinstep (a) comprises adding to the aluminum alloy between 5% and 20%silicon carbide nanoparticles by weight and milling the mixture at 200rpm for between 5 hours and 20 hours.
 9. The method of making a highperformance aluminum nanocomposite according to claim 7, wherein thesintering step is carried out at a temperature of between 400° C. and500° C.
 10. The method of making a high performance aluminumnanocomposite according to claim 7, wherein step (a) comprises addingthe aluminum alloy in powder form with an average particle size of 40microns.
 11. The method of making a high performance aluminumnanocomposite according to claim 7, wherein the ball mill comprises aplanetary ball mill.
 12. The method of making a high performancealuminum nanocomposite according to claim 7, wherein the aluminum-basedmetal alloy comprises 40 micron particles of an alloy of aluminum,silicon, and magnesium
 13. The method of making a high performancealuminum nanocomposite according to claim 7, wherein the aluminum-basedmetal alloy comprises Al-7Si-0.3Mg.
 14. The method of making a highperformance aluminum nanocomposite according to claim 7, wherein thealuminum-based metal alloy comprises Al-12Si-0.3Mg.
 15. A method ofmaking a high performance aluminum nanocomposite, comprising the stepsof: milling a mixture of particles of an Al—Si—Mg alloy having anaverage particle diameter of 40 μm and particles of silicon carbidehaving a diameter of between 20 nm and 40 nm in a planetary ball millfor about 20 hours in order to form a reinforced aluminum nanocomposite;and sintering the reinforced aluminum nanocomposite at a temperature ofabout 500° C. in order to consolidate the reinforced aluminumnanocomposite while retaining the reinforced aluminum nanocomposite in anano-crystalline structure.
 16. The method of making a high performancealuminum nanocomposite according to claim 15, wherein the siliconcarbide particles comprise between 5 wt % and 20 wt % of the mixture.17. The method of making a high performance aluminum nanocompositeaccording to claim 15, wherein the Al—Si—Mg alloy comprisesAl-7Si-0.3Mg.
 18. The method of making a high performance aluminumnanocomposite according to claim 15, wherein the Al—Si—Mg alloycomprises Al-12Si-0.3Mg.